Experimental investigation of a figure-eight fiber laser with a symmetrical NOLM and highly twisted fiber in the loop

We report experimental results that demonstrate the mode-locking operation of a figure eight fiber laser based on a symmetrical nonlinear optical loop mirror (NOLM) with a twisted low-birefringence fiber in the loop. The mode-locking operation is achieved by nonlinear polarization rotation in the NOLM, where the counter-propagating beams accumulate a differential nonlinear phase shift when they have different polarizations. By adjusting a single quarter wave (QW) retarder in the NOLM it was possible to change the NOLM transmission behavior from a maximum to a minimum at low input power. Self-starting mode-locking operation was found for only specific settings of the QW retarder. The pulse repetition frequency was around 0.8 MHz. The mode-locked laser operated stably for hours and the adjustment procedure was straightforward. We achieved stable generation of picosecond pulses with milliwatts of average output power.